Laser marking systems have been in existence as early as 1971 for marking indicia on surfaces of articles. A major use of laser marking of articles is for marking an article or product or a product package particularly with respect to high volume manufacturing lines, so as to take advantage of marking these goods "on-the-fly". This type of marking provides data about the product, such as, date of manufacture, shelf life, factory origin, model and/or serial number, product tracking and the like. The use of lasers to provide marking indicia is preferred since it does not generally affect the integrity of the article or product or its packaging and the marked indicia is not easily removable.
An example of traditional laser marking systems are cw or pulsed CO.sub.2 lasers and yttrium aluminum garnet (YAG), e.g., Nd:YAG lasers where the marking is accomplished by the heat of the applied laser beam. The wavelengths of the pulses produced by these systems are within the visible or infrared spectrum. A pattern or indicia to be marked is formed by using a mask through which the laser beam passes or by a focused laser beam which is moved or scanned to produce the desired indicia or pattern. Such lasers are also employed for engraving, soldering and welding wherein, the case of marking, the surface layer of the material is melted, ablated or vaporized to produce discernible indicia or pattern. Also, this type of article marking may be accomplished by use of a chemical reaction at the article surface to be marked where certain coating agents on the surface of the article, which may be visibly transparent, but undergo a visible contrast change under the influence of a laser beam or laser pulses.
CO.sub.2 lasers have been principally employed for marking plastic surfaces, such as IC packages. The laser beam from the laser is directed through a copper stencil to form the indicia on the plastic surface. However, due to the shrinkage of IC packages over the years, CO.sub.2 lasers, in many cases, are no longer suitable since high quality indicia with good visibility is no longer satisfactory for this particular application. However, low cost, lower marking quality CO.sub.2 systems employing low cost X-Y galvanometer devices are still employed for applications not requiring high quality marking.
YAG lasers are extensively employed today for IC package marking as well as many other marking applications. YAG lasers have shorter wavelengths of operation permitting the marking of indicia on harder surfaces, such as ceramic material. The beam in the YAG marking systems is steered or scanned in one, two or three dimensions by means of a pair of displaceable mirrors mounted for rotation to displace a laser beam in orthogonal directions to form a two-dimensional scan of the beam on the surface to be marked, such as, for example, a X-Y galvanometer device or a X-X galvanometer device operated under computer control. Examples of two-dimensional scanners are disclosed in U.S. Pat. Nos. 5,225,923; 5,329,090; 5,719,372; and 5,724,412. Indicia is scribed onto the surface of an article to be marked with fine resolution and marking clarity on comparatively, smaller surfaces, such as in the case of smaller IC packages. A specific example of a YAG laser system for this type of marking is the scanning Nd:YAG laser called the Laser Marker SL475E, manufactured by NEC Corporation of Japan. The marking parameters of this system are as follows: (1) Laser Oscillator: SL114K, (2) Laser Type: cw Nd:YAG laser, (3) Output: 50 W or above, (4) Number of Marked Characters: 40, (5) Marking Method: One stroke or vector, (6) Power at Marked Surface: 1 W, (7) Scanning Speed: 100 mm/sec., (8) Bite Size: 30 .mu.m; and (9) Q-Switch Frequency: 3 kHz.
The disadvantage of these CO.sub.2 and YAG laser marking systems is the need in most instances for separate, expensive refrigerated chillers or water cooling units and corresponding cooler controller and power supply to maintain cooling of the cw operated laser diode arrays for pumping the YAG rod or cw operated CO.sub.2 marking lasers. The chillers are required in CO.sub.2 marking lasers due to the low efficiency in converting lamp pump light into a cw laser output.
Further, the modulation of these marking lasers is generally accomplished by means of modulating their optical output beams, such as with an acusto-optic modulator, to produce appropriate pulses for forming marking strokes or vectors that, together, form intelligent indicia on the article surface. As a result, as much as 20% to 30% of the power in the modulated output is lost due to this type of external modulation. The cw operation of these types of lasers is a waste of energy, requires continual maintenance of the lasers, and reduces their overall lifetime utility. In the pulse mode, there is a large pulse-to-pulse variation in YAG marking lasers, as they lack uniformity in the energy applied to the marking surface. Moreover, the external modulator, beside its high loss, does not last long in the field and needs to be replaced, and is an added and continuing cost to the laser marking system, along with its RF driver. Further, the YAG laser systems used for marking require first pulse suppression, i.e., when the laser is turned off the light has to be "bled off". Also, these systems with their associated cooling units and large power supplies and large laser head takes up a consider amount of floor space in a manufacturing facility.
What is needed is a less expensive marking laser system that provides marking "power-on-demand", i.e., is not continually required to be continuously pumped for accomplishing the marking process, and taking up minimal floor space.
It is a principal object of this invention to provide a fiber laser system that provides indicia marking power-on-demand.
It is an object of this invention to provide a laser-pumped fiber laser marking system that is more compact and smaller in size than previous laser systems for marking surfaces to produce visible indicia on the surface.
It is another object of this invention to provide a laser marking system requiring no first pulse suppression.
It is another object of this invention to provide a first high power laser marking system employing a double clad fiber as the marking laser wherein its optical power output is modulated to form the marking indicia by modulation or switching ON and OFF of its pump laser, e.g., a semiconductor laser diode source.
It is a further object of this invention to provide a laser marking system that achieves high powers for surface marking accomplished with shallow surface depth significantly less than about 27 .mu.m.